1. Field of the Invention
The present invention relates to an ultrasonic sensor, especially to a waterproof ultrasonic sensor used for a back sonar and corner sonar of automobiles.
2. Description of the Related Art
Recently, demands for waterproof ultrasonic sensors for sensing short distance objects that come close to the sensor are increasing in the application field of ultrasonic sensors.
An ultrasonic sensor with a construction as shown in FIG. 17 has been used for the purposes described above. The ultrasonic sensor 51 has a construction in which a piezoelectric vibration element 53, on both main faces of which element electrodes (not shown in the drawing) are formed, is adhered at the inside of a vibration part (bottom face) 54 of a floored cylindrical case 52a that is integrally formed of a metal, such as aluminum, by a cylinder part 55 and the vibration part 54. Electrical connection from the element electrode formed on the piezoelectric vibration element 53 to the outside of the case 52a is achieved via input-output terminals 58a and 58b. The input-output terminal 58a is connected to the element electrode formed on the main face at the side (top face side) not making contact with the vibration part 54 of the piezoelectric vibration element 53 by, for example, soldering. The input-output terminal 58b is connected to a prescribed position of the metal case 52a which is in electric connection with the element electrode formed on the main face at the side (bottom face side) making contact with the vibration part 54 of the piezoelectric vibration element 53 by, for example, soldering. The construction as described above allows electrical connection from the element electrodes to be formed with the input-output terminals 58a and 58b.
A soft and fine copper wire is used for the wiring material of the input-output terminals 58a and 58b. If a highly rigid wiring material, such as a lead frame comprising, for example, an iron-nickel alloy is used for the input-output terminal, then the input-output terminal is inserted from the opening of the cylindrical case 52a toward the vibration part 54 to achieve electrical connection by pressing the tip of the input-output terminal into contact with the piezoelectric vibration element 53. However, if the piezoelectric vibration element is pressed down with the input-output terminal, it results in inhibition (restriction) of naturally required vibration of the element 53. Moreover, when a highly rigid wiring material makes a press-contact with the piezoelectric vibration element 53, vibration of the piezoelectric element leaks out of the inner/outer part of the case through the input-output terminal--so called "leaky vibration"--causing deterioration of reverberation characteristic of the ultrasonic sensor. Therefore, a soft and fine wire has been used for the input-output terminal for these reasons.
A vibration suppressing material such as a silicone resin (not shown) is usually inserted into the empty space inside of the cylindrical case of the foregoing ultrasonic sensor 51.
The ultrasonic sensor with the construction as described above operates as follows. Firstly, a driving voltage is applied to the input-output terminals 58a and 58b to allow the piezoelectric vibration element 53 to vibrate. This vibration forces the vibration part 54 of the floored cylindrical case 52 to vibrate, emitting an ultrasonic wave toward the direction indicated by the arrow in FIG. 17. After a prescribed time interval, the ultrasonic wave reflected back from the sensed object arrives at the piezoelectric vibration element 53 via the vibration part 54 and is converted into a reflection signal, followed by output of output signals from the input-output terminals 58a and 58b. Then, the time interval from application of the driving voltage through the output of the reflection signals is detected, thereby measuring the distance between the sensor and the sensed object.
Conventional ultrasonic sensors as described above have the following problems. In the foregoing ultrasonic sensor, the floored cylindrical case 52a is integrally constructed with the cylinder part 55 and the vibration part 54. While such an integrated case may be usually produced by machining, the inner diameter of the cylindrical case and the thickness of the vibration part are liable to variation in dimensions. Variation in dimensions of the inner diameter and thickness so greatly affect such characteristics of the ultrasonic sensor, such as resonance frequency, sensitivity and reverberation, that it becomes difficult to effectively produce the ultrasonic sensors with uniform characteristics, resulting in poor productivity. The machining process also brings about increases in cost of the ultrasonic sensor because of the expensive material employed.
A soft and fine copper wire is used for the input-output terminals 58a and 58b in the ultrasonic sensor with the foregoing construction from the view point of not adversely affecting the required vibration and reverberation characteristics of the vibration element. However, handling of the fine and soft input-output terminals is difficult. Also, it is very difficult to position the solder contact site against the piezoelectric vibration element. Consequently, the contact site becomes nonuniform among respective products thereby resulting in variation of the resonance frequency and sensitivity characteristic among the ultrasonic sensors. Therefore, effective production of an ultrasonic sensor with a uniform characteristic becomes difficult, resulting in poor mass productivity. Further, there is a possibility that the fine input-output terminals may be broken due to the effect of vibration of the piezoelectric vibration element, thereby adversely affecting the reliability of the electrical connection.